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distinguish two regions between flap and film. In one
region, the film is at a distance of 5 mm from the
applicator, and in the other region at a distance of 7 and
9 mm (5mm of PMMA plus 2 or 4mm air gap
respectively).Two different treatment plans have been
designed, in the first one the source stops in the center of
the spheres and in the other one at the edge, to compare
the difference between dwell positions. The dwell times
are set to get the dose distribution as uniform as possible,
prescribing 6 Gy at a depth of 5 mm.
Results
Results obtained are shown in table 1. Underdosage is
observed, produced by air layers, ranging from 4.8% to
10.8% when dwell positions are at the center of the
spheres, and from 6.2% to 11.8% when dwell positions are
at the edge of the spheres, with 2 and 4 mm air gap
respectively.
Conclusion
In view of the results obtained, it can be concluded that
several layers of air between the applicator flap and the
skin can lead to considerable variation in dosimetry, which
may involve the loss of effectiveness of treatments with
this type of applicators. Thus, utmost care is required
during the placement of the flap to minimize the error due
to the air gap, therefore avoiding an underdosage in the
volume to be treated.
PO-0945 Pretreatment verification for brachytherapy
G. Fonseca
1
, M. Podesta
1
, M. Bellezzo
1
, B. Reniers
2
, F.
Verhaegen
1
1
Maastro Clinic, Physics, Maastricht, The Netherlands
2
University of Hasselt, NuTeC, Hasselt, Belgium
Purpose or Objective
Individual plan QA is not performed in brachytherapy
mostly due to the large uncertainty associated with dose
measurements. Traditional setups require precise and
accurate positioning, and therefore usually laborious
procedures to detect anything other than large
discrepancies with an unclear distinction between source
or detector mispositioning. This study evaluates the use of
an Electronic Portal Imaging (EPID) to verify the treatment
plan.
Material and Methods
The EPID panel response was characterized with an High
Dose Rate (HDR) Ir-192 source. A robotic arm was
employed for positioning within a water tank (Figure 1a)
assuring 0.2 mm accuracy during the calibration, which
covered a clinically relevant range for the distance
between the source and the panel (from 6 up to 25 cm).
Experiments were performed with an acquisition rate of
6.7 fps for a single catheter and for a gynecological
cylinder applicator (Figure 1b) with 5 catheters. Inter-
dwell distances of 2 and 5 mm were employed and the
experiments performed for source activities between 5
and 10 Ci. The EPID response is proportional to the source
activity so it is possible to obtain the activity by sending
the
source
to
pre-defined
dwell
position.
Results
3D Cartesian coordinates can be obtained with 0.2 mm
accuracy using a single EPID panel. The panel can clearly
identify dwell positions 2 mm apart even with the catheter
at 24 cm distance (Figure 1c) from the panel. Absolute
coordinates can be obtained by adding reference points
(representing the corners of the water phantom) in the
treatment plan that can be related with the position of
the water phantom over the panel during the experiments.
An
in-house
developed software compares all dwell
positions/times against the treatment plan. The software
can also monitor the sequence of the treatment
identifying the afterloader channel connected to each
catheter. Therefore, it is possible to detect catheter
misplacements, swapped transfer tube connections,
wrong dwell times and/or positions and also verify the
source activity.
Conclusion
This work describes an experimental system that can be
implemented in the clinic
providing experimental pre-
treatment verification that is not currently available. This
method provides several advantages when compared
against other dosimeters such as films or MOSFETs as it
combines a 2D dosimeter, which has an online response.
Our system can detect several problems that would be
unnoticed during the treatment if only traditional QA is
performed.
PO-0946 Entropic model for real-time dose
calculation: I-125 prostate brachytherapy application.
G. Birindelli
1
, J.L. Feugeas
1
, B. Dubroca
1
, J. Caron
1,2
, J.
Page
1
, T. Pichard
1
, V. Tikhonchuk
1
, P. Nicolaï
1
1
Centre Lasers Intenses et Applications, Interaction-
Fusion par Confinement Inertiel- Astrophysique,
Talence, France
2
Institut Bergonié Comprehensive Cancer Center,
Department of radiotherapy, Bordeaux, France
Purpose or Objective
This work proposes a completely new Grid Based
Boltzmann Solver (GBBS) conceived for the description of
the transport and energy deposition by energetic particles
for brachytherapy purposes. Its entropic closure and
mathematical formulation allow our code (M
1
) to calculate
the delivered dose with an accuracy comparable to the
Monte Carlo (MC) codes with a computational time that is
reduced to the order of few seconds without any special
processing power requirement.